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512                              CHEMICAL ENGINEERING
The slope of the curve is such that no decided drop in the fusion temperature occurs when the molecular proportion of the two constituents is changed within reasonable limits. It is evident, therefore, that the spinel offers a type of refractory of very promising usefulness, especially under conditions requiring basic refractories.
In producing this refractory the purer grades of magnesite and bauxite should be used and no binder such as clay can be employed. Wherever possible an organic binder like linseed oil or a small amount of low-burnt magnesia may be used. The Bureau of Standards has made crucibles of magnesia-spinel successfully without the addition of any inorganic binder by grinding a portion of the material very fine. According to the purity of the magnesite and bauxite the calcining temperature should vary from cone 20 to 30 and the firing of the articles must be carried to at least cone 20. The required proportion of magnesite to bauxite corresponds to the ratio 84 :102 referred to the MgC03 and A1203 contained in these materials.
Carbon Refractories.ŚClay materials containing carbon in some form have been in use for a long time. Carbon itself must be considered a refractory of high grade which can be used wherever the possibility of continuous oxidation is excluded.
Brick made from a mixture of crushed coke and clay or from coke bonded by means of tar have been made many years ago. These materials are usually hand molded, dried and fired in muffles with as complete exclusion of air as possible. The carbonization of the tar cements together the coke particles to a compact mass. The coke may also be replaced by graphite. Refractories of this type combine good heat-resisting power with excellent thermal conductivity.
Mixtures of clay and graphite are used very largely in the crucibles employed in the melting of metals. Here the graphite contributes toward the refractoriness and heat conductivity of the mass, prevents the oxidation of the metal, and produces a smooth surface for pouring.
The function of the clay is that of a bonding material, which makes possible the shaping of the crucible and the cementing together of the graphite flakes. At the same time, it covers the particles, and thus protects them from oxidation. In order to be most effective in regard to this point, it is necessary that the clay contract and become dense at as low a temperature as is consistent with the required refractoriness.
As to the real nature of graphite we are still in the dark. It is crystalline, rhombo-hedral, and stable at the highest temperatures. Its great softness, flaky structure, and feeble metallic luster are characteristic. Its specific heat varies in the natural and artificial varieties and according to the purity of the materials. Values have been determined for Ceylon graphite of from 0.174 to 0.2019 and for blast-furnace graphite of from 0.166 to 0.1970. The specific heat increases with temperature and has been found to be 0.467 at 977░C. The coefficient of thermal conductivity of graphite (gram calories per degree Centigrade through 1 cm.3) is 0.0141, and hence, five times that of burned clay. The linear coefficient of expansion is 0.00000786 at 40░C. The excellent electrical conductivity of graphite is well known.
The principal foreign sources, of graphite are: Ceylon, Canada, Mexico and Madagascar. In the United States the chief sources of the mineral are to be found in Alabama, New York and Pennsylvania. Artificial graphite is produced in the electric furnace, and a soft variety known as "kish " is the product of the blast furnace. Possible sources of carbon which may be used in this connection are retort graphite formed in gas retorts and the carbon resulting from the decomposition of hydrocarbons.
In European practice both coke and retort graphite have been admixed with the natural graphite for making crucibles. The practice is not desirable for steel melting,